The invention relates to an apparatus and method for heating a workpiece of an inductively heatable material.
Turbine buckets are often provided with a coating which has special properties; for example, it is extremely hard or serves only for thermal insulation. In order that these coatings, which are applied, for example by EB or PVD or sputtering methods, may be bonded to the surface of the turbine buckets, these turbine buckets must be heated to a specific temperature.
Heating turbine buckets to a specific temperature is problematical especially because a turbine bucket is not a symmetrical or in any way uniform work piece, but has a base of great mass and a blade of low mass. On account of this basic structure of a turbine bucket the uniform heating of all parts of a turbine bucket can be achieved only with difficulty.
It is already known to heat turbine buckets by radiant heating, for example with infrared heaters. However, with radiant heating only a limited power-area ratio can be achieved, with the result that a relatively long time is needed to raise heavy gas turbine buckets, for example, to temperature.
To eliminate this disadvantage, turbine buckets have already been heated by means of electron beams, which lead to a higher power-area ratio and, if an appropriate pattern of movement of the electron beams is selected, they produce uniform heating as well.
In the case of electron beam heating, it is a disadvantage that a complex electron gun with a likewise complex deflection system is required. Moreover, in electron beam heating the energy which leads to the heating of the workpiece can be applied only to the surface of the workpiece. Therefore, due to limited thermal conductivity it takes a relatively long time before the heat has penetrated into the depth of the workpiece.
The heating of materials of good electrical conductivity, especially metals, by means of alternating magnetic fields has long been known in the form of so-called xe2x80x9cinductive heatingxe2x80x9d (Meyers Enzyklopxc3xa4disches Lexikon, Vol. 12, Hf-Iz, page 566). The material or workpiece is placed inside of an often liquid-cooled working coil through which an alternating current flows. The alternating magnetic field that is formed produces eddy currents in the workpiece and their Joulean heat heats the material.
To preheat the surface of a body for treatment and obtain uniform carburization, it is also known to superimpose an alternating magnetic field on a rotating magnetic field and thus produce induction heating (JP 0062297453 AA).
It is furthermore known to use induction heating to bring the temperature to 3300xc2x0 C. within 20 seconds (U.S. Pat. No. 5,993,058). In this induction heating a susceptor is used, which is brought into a variable magnetic field which exerts an electromotive force. The induced currents (or eddy currents) which flow through the susceptor are converted to heat. A specimen to be heated is then heated by the radiation of the susceptor.
Lastly, another apparatus and a method for coating metal objects from a metal vapor are known, in which the object is passed through an induction coil in order to preheat a portion of the object""s surface to a specific temperature (EP 0 094 759 A2). The preheated portion is then bathed in a stream of metal powder so that the metal powder deposits itself thereon. The object thus treated passes then through a second induction coil which heats it and the metal powder to a certain temperature so that the metal powder fuses with the object.
The invention is therefore addressed to the problem of permitting a rapid heating of workpieces, especially turbine blades, before and/or during a coating process.
The problem is solved by the present invention.
The invention thus relates to an apparatus and a method for heating a workpiece of metal, for example a turbine blade. Such a turbine blade consists of a base of great mass and a low-mass wing. To enable the base and the wing to be heated appropriately, an induction coil is provided which surrounds the base and wing. By the appropriate choice of the frequencies of the alternating currents that flow through the induction coil and/or by the insertion of a susceptor between the turbine blade and the induction coil, the base and wing can be heated in an appropriate manner.
The advantage achieved with the invention consists especially in the fact that the heat does not have to penetrate from the exterior into the workpiece by thermal conduction, but is formed within the workpiece. Thus the heat can be distributed uniformly or non-uniformly through the workpiece, say by varying the pitch of the coil or the coil diameter according to the longitudinal axis of the coil. The heat distribution can also be controlled by various screening around the workpiece or by selecting various frequencies.